After hydrocarbons have been removed from the ground, the fluid stream (such as crude or natural gas) is transported from place-to-place via pipelines. It is desirable to know with accuracy the amount of fluid flowing in the stream, and particular accuracy is demanded when the fluid is changing hands, or “custody transfer.” Ultrasonic flow meters may be used to measure the amount of fluid flowing in a pipeline, and ultrasonic flow meters have sufficient accuracy to be used in custody transfer.
In an ultrasonic flow meter, ultrasonic signals are sent back and forth across the fluid stream to be measured. One of the parameters sensed in determining fluid flow is arrival time of an ultrasonic signal at a transducer. However, because of noise in the fluid system, and inherent shortcomings in the electronic systems of the ultrasonic meter, it is difficult in some situations for the electronics of the meter to consistently select the same feature of a detected acoustic signal to signify arrival time. More particularly, in some situations the selected feature of the detected ultrasonic signal used to signify the arrival time may be off by one or more cycles of the ultrasonic signal from the desired feature. Where the feature selected by the electronics to signify arrival of the ultrasonic signal is different than the desired feature, “cycle skip” is said to have occurred.
The situation is further complicated by the fact that many ultrasonic meters have multiple transducer pairs, each transducer pair sending and receiving ultrasonic signals. The feature of the detected ultrasonic signal for a first transducer pair may be off a cycle or more before the desired feature, and the feature of a second transducer pair may be off a cycle or more after the desired feature. In fact, in an ultrasonic meter having four transducer pairs, and considering only correctly identifying the desired feature, cycle skip of one cycle before the desired feature, and cycle skip of one cycle after the desired feature (three possibilities), there are 34 or 81 different cycle skip configurations that could occur. Given the number of possible cycle skip configurations, identifying the cycle skip configuration may be difficult, particularly where the processing power of the electronics is limited.